322 Our Surroundings 



Respiration. In respiration the amoeba, like the paramecium, 

 simply absorbs air from the water which surrounds it, through 

 its thin cell wall, and in the same way throws off the carbon 

 dioxide resulting from the oxidation of the protoplasm and the 

 food. 



Food-taking. The amoeba has no mouth. It secures its food 

 by the action of its false feet which surround the food and enclose 

 it in the protoplasm. 



Digestion, Absorption, Assimilation and Excretion. 

 Enzymes secreted by the protoplasm render the food soluble as it 

 circulates in the cell body. The nourishing elements of the food 

 are rapidly absorbed and assimilated by the animal, and the wastes 

 resulting from oxidation are excreted by means of vacuoles, as in 

 the paramecium. 



Reproduction. Like the paramecium, the amoeba reproduces 

 by cell-division, or by cell-union. The fact that it multiplies 

 rapidly when kept under suitable conditions seems to indicate that 

 it undergoes division frequently. 



When conditions for life are unfavorable, the 'amoeba some- 

 times surrounds itself with a membranous wall and apparently 

 becomes lifeless, like an insect in the cocoon stage. On the return 

 of favorable conditions, however, it resumes its life func- 

 tions. 



Kinds of Protozoa. There are many kinds of protozoa, or 

 one-celled animals. Nearly all live in water or in the bodies of 

 other animals. Some protozoa are useful, others are harmful. 



Useful Protozoa. Vast numbers of protozoa exist in bodies 

 of water, and are useful as a source of food for fish. Fish in 

 turn supply food for man. Again, the shell-bearing varieties pro- 

 duce beds of limestone rock. When they die, their shells settle 

 to the bottom of the ocean and form masses of a grayish sub- 

 stance. This has been found in dredgings and is composed largely 

 of the shells of protozoa. Chalk is made up of the shells of these 

 animals. The presence of large deposits of chalk on land is evi- 

 dence that the region was once the bed of the ocean. Many 

 protozoa are of some importance as scavengers, since they eat 

 particles of dead organic matter. 



